論文

An allotrope of Si, Si136, is a promising semiconductor for photovoltaics due to its direct bandgap. Si136 is synthesized via Na extraction from Na24Si136. In this study, 93 at.% of Na extraction from a single crystal of Na24Si136 was achieved using high-pressure-annealed zeolite as an ion absorber. The residual amount of Na is the least record among Na extraction from a single-crystalline Na24Si136. Na diffusion between the grains of amorphous phases of zeolite was suggested to be improved due to high-pressure annealing.

Na-free Si clathrates consisting only of Si cages are an allotrope of diamond-structured Si. This material is promising for various device applications, such as next-generation photovoltaics. The probable technique for synthesizing Na-free Si clathrates is to extract Na+ from the Si cages of Na24Si136. Vacuum annealing is presently a well-known conventional and effective approach for extracting Na. However, this study demonstrates that Na+ cannot be extracted from the surface of a single-crystalline type-II metallic Si clathrate (Na24Si136) in areas deeper than 150 µm. Therefore, a novel method is developed to control anisotropic ion diffusion: this is effective for various compounds with a large difference in the bonding strength between their constituent elements, such as Na24Si136 composed of covalent Si cages and weakly trapped Na+. By skillfully exploiting the difference in the chemical potentials as a driving force, Na+ is homogeneously extracted regardless of the size of the single crystal while maintaining high crystallinity. Additionally, the proposed point defect model is evaluated via density functional theory, and the migration of Na+ between the Si cages is explained. It is expected that the developed experimental and computational techniques would significantly advance material design for synthesizing thermodynamically metastable materials.

We investigate the Al substitution effect for HoB2 to tune the Curie temperature (TC). The HoB2−xAlx samples with nominal x (xnom) of 0, 0.1, 0.3, and 0.5 are characterized by powder X-ray diffraction measurements, EDS analysis and magnetization measurements. The HoB2−xAlx samples with xnom of 0.1, 0.3, and 0.5 show two ferromagnetic transitions at TC1 of ∼15 K and TC2 of >32 K. The TC1 is identical to the TC of HoB2 and does not change with Al substitution. The TC2 is similar to the TC of HoAl2 and varies with Al content. In an EDS analysis, the inhomogeneous elemental distribution is observed in Al-substituted HoB2, where the Al-rich regions exist. Our results suggest that the Al added to HoB2 does not form a solid solution, but rather leads to the phase segregation into stoichiometric HoB2 and B-substituted HoAl2. The xnom = 0.5 sample exhibits the maximum |ΔSM(μ0H:5 T→0 T)| of 32.1 J kg K−1 at TC1 and a small kink of |ΔSM| at TC2. Because the |ΔSM| exhibits a maximum at TC1, stoichiometric HoB2 mainly contributes to the maximum of |ΔSM|.